The present invention relates to a developing device for use in a copier or similar image forming equipment and using a one-component developer, i.e., a toner.
It is a common practice to provide a developing device for the above application with a developing roller, a toner hopper storing a toner to be fed to the developing roller, and an agitator for driving the toner out of the toner hopper toward the developing roller. The developing roller, toner hopper and agitator are usually arranged side by side substantially in the horizontal direction. Hence, in a color copier or a color printer, for example, having three or four developing devices, the developing devices have to be arranged side by side substantially in the horizontal direction. This prevents various devices or units constituting, for example, a color copier from being freely laid out and, therefore, needs a large size photoconductive element, increasing the overall size of the copier. Moreover, it is likely that the toner is scattered around via the opening of the developing device to smear the equipment and to enter the following developing device which stores a toner of another color.
To eliminate the above problem, a developing device having a developing roller, toner hopper and so forth arranged in the vertical direction may be disposed below a photoconductive element to supply a developer upward to the element, as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 112753/1978, 185052/1982, and 223158/1983 which will respectively be referred to as Prior Art 1, 2 and 3 for convenience.
The developing device of Prior Art 1 is elaborated to surely supplement a toner, miniaturize the configuration, and promote free layout of a developing roller and a toner tank. A flexible thin member is constantly biased to in turn bias a toner stored in the toner tank against the developing roller. As a result, the toner is semiforcibly supplied to the developing roller until it has been fully consumed. This successfully eliminates so-called bridging of toner.
Prior Art 2 proposes a non-contact type developing device capable of causing a toner of predetermined polarity and potential to form a uniform thin layer by use of a pair of developing rollers. Specifically, a toner is deposited in a great amount on one developing roller. Only part of this toner having desired polarity and charge is transferred from the developing roller to the other developing roller by an electric force to form an about less than 300 microns thick layer thereon. The thickness of the toner layer is controlled on the basis of the peripheral speed of each roller or the gap between the rollers or the bias for development.
Further, Prior Art 2 teaches a developing device which allows a toner to be fed to a developer carrier in any desired direction so as to enhance design freedom. Specifically, the device has a bag implemented by an elastic thin film and in which a developer is packed against the elasticity of the bag. As the elastic bag contracts, the developer is urged against the surface of a developer carrier.
However, the problem with Prior Art 1 is that the toner supplied to the developing roller is not sufficiently charged and, therefore, apt to come off from the roller due to the small electrostatic force between the toner and the roller. As a result, much of the toner is scattered around before reaching a frictional charging member, contaminating the surroundings. The leakage of the toner is aggravated since a pressure constantly acts on the opening of the developing device.
Prior Art 2 selects a predetermined part of the toner out of the toner layer whose charge has not been fully controlled and, therefore, causes the toner with undesired charges to sequentially accumulate. It follows that charge control matching the charging ability has to be executed from time to time, resulting in the need for exclusive sensing means. Moreover, despite the control of the charge of the toner to a predetermined one, image quality cannot be maintained constant since the toner repetitively discriminated by electric means has changed the particle size and characteristic thereof.
Prior art 3 has a drawback that the contraction of the bag acts in various directions since the toner is packed in the bag against the elasticity of the bag. Specifically, the device of Prior Art 3, like the device of Prior Art 1, constantly exerts a pressure on the opening of the bag. This makes it difficult to seal the opening of the bag and that of the device. Defective sealing would cause the toner to fly out of the bag and device. Particularly, when an application roller is disposed in the bag, it aggravates the movement of the toner and, therefore, the leakage of the toner. Moreover, when the bag is scratched or otherwise damage, it will break up to scatter the toner. In addition, the pressure constantly acting on the toner stored in the bag causes the toner to cohere, thereby lowering image density or otherwise degrading image quality.
The problem with all of the conventional developing devices described above is that since they supply the toner upward to the photoconductive element, the toner cannot be transported over a substantial distance without increasing the load for structural reasons.